Just curious does anyone here own the five volume set?  I see some discussion about the merits of MCO, but isn't the ECO the standard opening reference even today?

A five volume book called the Encyclopedia of Chess Openings (in French) that isn't ECO?

No clue. 

I own the five volumes of ECO. 

The French title of ECO is indeed 

L'Encyclopédie des ouvertures d'échecs

And I thought it was something by Chopin.

So how is it?  How does one "use" the ECO?

Presumably as one would use a dictionary. I can't imagine a whole lot of utility being derived from physical posession of it.

I own the printed five volumes. I would use them more often if I did not also own the CD version and a licensed copy of Chess Informant Expert to facilitate viewing.

It rivals ChessBase (and access to five million+ games) in my opening study.

I compare some of the merits of each in http://chessskill.blogspot.com/2009/08/encyclopedia-of-chess-openings.html 

I bought my five volumes long before it was available electronically. Today, I mostly use the companion Encyclopedia of Chess Middlegames/Combinations which was part of the package. The cost of the entire six volumes in hardback, nearly pristine condition? $100! Even now, it would be a steal at that price! (The book seller had placed it in the foreign language section rather than the chess section, otherwise it would have disappeared long before I got there.)

I used to use it in correspondence chess and to refine my opening repertoire. Now, it mostly gathers dust.

SmyslovFan: you got a deal. I paid $30 per volume, and that was in the late 1990s through 2001. No bookstore in my area had the books, so I bought them from the USCF. Now, of course, Amazon has affected the prices, but they have incorrect information so severe that a buyer cannot be certain what will arrive in the box.